1. Field of the Invention
The present invention relates to a bearing washer for a thrust bearing and to a thrust bearing, and more particularly to a bearing washer for a thrust bearing manufactured by quench hardening and used without being subjected to grinding after quench hardening and to a thrust bearing including the bearing washer.
2. Description of the Background Art
For example, a thrust needle roller bearing including a needle roller, a cage and a bearing washer is structured such that the needle roller and the bearing washer are in line contact. Therefore, the thrust needle roller bearing is advantageous in achieving high load carrying capacity and high rigidity even with a small projected area of the bearing. Accordingly, the thrust needle roller bearing is suitable as a bearing used under a severe condition such as under insufficient lubrication or high-speed revolution, and employed, for example, in a compressor for an air-conditioner for an automobile, an automatic transmission, a manual transmission, a continuously variable transmission, a transmission with an actuator, a brake-by-wire, a differential gear, a transfer, and an outboard motor.
For example, in a compressor for an air-conditioner for an automobile, oil that is used has low viscosity and an amount of oil has been reduced in order to improve compressor performance (cooling performance). The thrust bearing is used under such a severe condition as under insufficient lubrication. As such, if differential slip of the roller is great, failure at an early stage due to surface damage such as surface-originating flaking may occur, and curing of such a defect has been demanded.
Meanwhile, when the thrust bearing is employed in an automatic transmission, a manual transmission, a continuously variable transmission, a transmission with an actuator, and a differential gear, from a viewpoint of energy saving, oil of low viscosity may be used or conventional oil to which an additive has been added may be used. The oil of low viscosity or the additive-containing oil is poorer than normal oil in lubricating performance in the bearing. Therefore, from a viewpoint of preventing surface damage such as surface-originating flaking, improvement in a state-of-the-art thrust bearing in which differential slip of the roller is great has been demanded.
In addition, the thrust bearing employed in a compressor for an air-conditioner for an automobile, an automatic transmission, a manual transmission, a continuously variable transmission, a transmission with an actuator, a brake-by-wire, a differential gear, a transfer, an outboard motor, and the like tends to be used under high load and its size also tends to be made smaller. Therefore, improvement also from a viewpoint of internally originating flaking caused by ordinary load-dependent rolling fatigue has been desired.
In view of the above, a long-life bearing effective against failure at the early stage due to surface damage such as surface-originating flaking and also against internally originating flaking caused by ordinary load-dependent rolling fatigue has been demanded.
In order to meet such a demand, a thrust needle roller bearing achieving longer life by improving a shape or the like of a cage of the thrust needle roller bearing so that an amount of passage of a lubricant per a unit time is increased has been proposed (see Japanese Patent Laying-Open No. 2002-70872).
From a viewpoint of longer life, it is also possible that camber/waviness of the bearing washer for the thrust bearing may cause a problem. Namely, if camber/waviness of the bearing washer is great, such a phenomenon that solely a part of a rolling contact surface of the roller serving as a rolling element is pressed against the bearing washer at the time of operation of the bearing (partial contact) takes place. Such partial contact may cause disruption in an oil film between the bearing washer and the roller serving as the rolling element. If the oil film is disrupted, the roller and the bearing washer come in metal-to-metal contact, and a temperature at that portion is raised. As a result, surface damage or surface-originating flaking is caused, which leads to shorter life of the bearing. In addition, if partial contact should occur, a contact pressure between the roller and the bearing washer may exceed a level expected in terms of design in a partial contact portion. In such a case, internally originating flaking caused by rolling fatigue occurs at an early stage, which results in shorter life of the bearing.
If camber/waviness of the bearing washer for the thrust bearing is great, noise or vibration at the time of operation of the bearing becomes large, which is quite problematic in an environment where operation sound should be low.
In order to address this problem, a method of subjecting an annular element corresponding to the bearing washer to prescribed forming in the cooling step in quenching of the annular element while a structure of the annular element is still in an austenite state has been proposed (see Japanese Patent Laying-Open No. 8-22585 1). This method serves to suppress strain of the annular element after quenching.
Moreover, a method of subjecting a ring member corresponding to the bearing washer to corrective tempering at a prescribed ratio has been proposed. This method serves to improve dimension accuracy of the ring member after completion of heat treatment (see Japanese Patent Laying-Open No. 9-256058).
In addition, a method of heating a ring member that has been subjected to cold working, in a state secured in a die, has been proposed. The ring member is thus subjected to sizing, and stress imposed by working is eliminated. Consequently, deformation of the ring member caused in subsequent heat treatment is suppressed (see Japanese Patent Laying-Open No. 11-43717).
Recently, however, a product using a thrust bearing as a part, such as a compressor for an air-conditioner for an automobile, an automatic transmission, a manual transmission, a continuously variable transmission, a transmission with an actuator, a brake-by-wire, a differential gear, a transfer, and an outboard motor has increasingly been sophisticated. Accordingly, further sophistication and higher precision such as longer life has been demanded in the thrust bearing that is used. In addition, in order to enhance cost-competitiveness of the product, an inexpensive thrust bearing has also been demanded.
Under such circumstances, durability of the thrust needle roller bearing disclosed in Japanese Patent Laying-Open No. 2002-70872 described above is insufficient, considering recent high demand on the thrust bearing. In addition, durability of the thrust bearing that achieves suppressed camber/waviness of the bearing washer with the manufacturing method disclosed in Japanese Patent Laying-Open Nos. 8-225851, 9-256058 and 11-43717 is also insufficient. Meanwhile, it runs counter to the demand for lower cost to improve durability with a method involving increased manufacturing cost or material cost.